1. Field of the Invention
The present invention relates to an apparatus for measuring the velocity of propagation of a pulse wave which is propagated through an artery of a living subject.
2. Related Art Statement
The blood pressure, degree of arterial sclerosis, peripheral resistance, etc. of a living subject can be estimated based on the velocity of propagation of a pulse wave which is propagated through an artery of the subject. There is known a pulse-wave propagation velocity measuring apparatus including a pair of pulse-wave sensors which are put on different locations on the skin of a living subject to press different portions of an artery of the subject via the skin and detect respective pulse waves from the artery. The measuring apparatus determines a pulse-wave propagation velocity based on the phase difference of the two pulse waves detected through the two sensors. The measuring apparatus is disclosed in, e.g., Unexamined Japanese Patent Application laid open for inspection purposes under Publication No. 60(1985)-220037.
The prior measuring apparatus measures the pulse-wave propagation velocity through the pulse-wave sensors being pressed against two portions of a superficial artery, such as carotid artery, radial artery, or dorsal pedal artery, that is adjacent to the skin of the subject. In this case, however, the time difference between the time of occurrence or detection of a heartbeat-synchronous pulse of one of the two pulses waves and the time of occurrence or detection of a corresponding heartbeat-synchronous pulse of the other pulse wave is small, because the length or distance between the two portions of the superficial artery is short. Thus, the time difference may not be relied upon for providing a very accurate pulse-wave propagation velocity.
It is therefore an object of the present invention to provide an apparatus for measuring a very accurate pulse-wave propagation velocity.
The above object has been achieved by the present invention. According to a first aspect of the present invention, there is provided an apparatus for measuring a propagation velocity of a pulse wave which is propagated through an artery of a living subject, the apparatus comprising an electrocardiographic-waveform detecting device which detects an electrocardiographic waveform from the subject, a pulse-wave sensor which is adapted to be worn on the subject and which detects the pulse wave from the subject, time-difference determining means for determining a time difference between a first periodic point relating to the detected electrocardiographic waveform and a second periodic point relating to the detected pulse wave, and propagation-velocity determining means for determining the propagation velocity of the pulse wave based on the determined time difference.
In the pulse-wave measuring apparatus in accordance with the first aspect of the invention, the time difference determined by the time-difference determining means contains a time duration when the pulse wave is propagated through the aorta directly connected to the heart of the subject. Therefore, the length of the artery through which the pulse wave is propagated is increased and accordingly the time difference is increased. In addition, since the diameter of the aorta is large and accordingly the velocity of the pulse wave being propagated through the aorta is decreased, the time difference is increased. Thus, the present apparatus provides a very accurate pulse-wave propagation velocity. Therefore, the degree of arterial sclerosis, or the blood pressure, of the subject can be estimated with high accuracy based on the thus determined pulse-wave propagation velocity.
According to a preferred feature of the first aspect of the invention, the pulse-wave sensor comprises a pressure sensor which is adapted to be pressed against a portion of the artery of the subject via a skin of the subject and which detects, as the pulse wave, a heartbeat-synchronous pressure oscillation which is transmitted thereto from the portion of the artery via the skin. In the case where the present invention is applied to a BP monitor apparatus including an identical pressure sensor for monitoring BP values of a subject, the pressure sensor is used for the two purposes, one for detecting a pulse wave and thereby measuring a pulse-wave propagation velocity and the other for monitoring BP values. The BP monitor apparatus having the pulse-wave propagation velocity measuring function enjoys a reduced production cost.
According to another feature of the first aspect of the invention, the pulse-wave sensor comprises an inflatable cuff which is adapted to be wound around a body portion of the subject, and a pressure sensor which detects, as the pulse wave, a heartbeat-synchronous pressure oscillation which is produced in the cuff. In the case where the present invention is applied to a BP measuring apparatus including identical cuff and pressure sensor for measuring a BP value of a subject, the cuff and pressure sensor are used for the two purposes, one for detecting a pulse wave and thereby measuring a pulse-wave propagation velocity and the other for measuring a BP value. The BP measuring apparatus having the pulse-wave propagation velocity measuring function enjoys a reduced production cost.
According to another feature of the first aspect of the invention, the pulse-wave sensor comprises a photoelectric sensor including a light source which emits a light toward a body portion of the subject, and a light detecting element which detects the light transmitted through, or reflected from, the body portion. In the case where the present invention is applied to a blood-oxygen-saturation measuring apparatus (e.g., so-called pulse oximeter) having an identical photoelectric sensor for measuring a blood oxygen saturation of a subject, the photoelectric sensor is used for the two purposes, one for detecting a pulse wave and thereby measuring a pulse-wave propagation velocity and the other for measuring a blood oxygen saturation. The oxygen-saturation measuring apparatus having the pulse-wave propagation velocity measuring function enjoys a reduced production cost.
According to a second aspect of the present invention, there is provided an apparatus for measuring a propagation velocity of a pulse wave which is propagated through an artery of a living subject, the apparatus comprising a blood-pressure measuring device which measures a blood pressure value of the subject, an electrocardio graphic-waveform detecting device which includes a plurality of electrodes adapted to contact a body surface of the subject and which detects an electrocardiographic waveform from the subject through the electrodes, a pulse-wave sensor which is adapted to be worn on the subject and which detects the pulse wave from the subject, time-difference determining means for determining a time difference between a first periodic point relating to the detected electrocardiographic waveform and a second periodic point relating to the detected pulse wave, propagation-velocity determining means for determining the propagation velocity of the pulse wave based on the determined time difference, and propagation-velocity modifying means for modifying the determined propagation-velocity value to a modified propagation-velocity value corresponding to a predetermined blood pressure value, based on the measured blood pressure value, according to a predetermined relationship between modified propagation velocity, and determined propagation velocity and measured blood pressure.
While the pulse-wave propagation velocity is influenced by the degree of arterial sclerosis, it is also influenced by the blood pressure and/or the pulse rate of the subject. However, the blood pressure (BP) values of the subject will change more or less in different measurements. Therefore, the propagation velocity measured by the prior apparatus may not directly be used as an index indicative of a degree of arterial sclerosis. In the pulse-wave propagation velocity measuring apparatus in accordance with the second aspect of the invention, however, the modified pulse-wave propagation velocity provided by the propagation-velocity modifying means can directly be used as an index indicative of a degree of arterial sclerosis. Thus, even if the BP or pulse-rate values obtained from the subject in different measurements may differ from each other, the present apparatus modifies each measured propagation velocity value to a modified propagation velocity corresponding to the predetermined BP value. Accordingly, a series of modified propagation-velocity values can directly be used as an index indicative of a time-wise change of the degree of arterial sclerosis of the subject.
According to a preferred feature of the second aspect of the invention, the propagation velocity measuring apparatus further comprises a pulse-rate measuring device which measures a pulse rate value of the subject, and the propagation-velocity modifying means comprises means for modifying the determined propagation-velocity value to the modified propagation-velocity value corresponding to the predetermined blood pressure value and a predetermined pulse rate value, based on the measured blood pressure value and the measured pulse rate value, according to the predetermined relationship between modified propagation velocity, and determined propagation velocity, measured blood pressure value, and measured pulse rate. Since the thus modified propagation-velocity value corresponds to both the predetermined BP and pulse-rate values, it is more accurate than a modified value corresponding to only the predetermined BP value, because the former value is free from influences from the fluctuation of the pulse rate whereas the latter value is subject to them.
According to another feature of the second aspect of the invention, the propagation velocity measuring apparatus further comprises coefficient determining means for determining a coefficient which is variable with the propagation velocity determined by the propagation-velocity determining means and with the blood pressure measured by the blood-pressure measuring device, and the propagation-velocity modifying means comprises means for modifying the determined propagation-velocity value to the modified propagation-velocity value corresponding to the predetermined blood pressure value, such that the modified propagation-velocity value is equal to a product of the determined coefficient and a difference between the predetermined blood pressure value and the measured blood pressure value. Thus, the modified propagation velocity is determined using the coefficient which is determined based on the propagation velocity and the diastolic BP value each obtained from the subject. Owing to the coefficient, the modified value is free from influences or fluctuations due to the differences of individual persons regarding the degree of arterial sclerosis. Therefore, the modified propagation-velocity values obtained from different subjects can be compared with each other for comparing the respective degrees of arterial sclerosis of those subjects.
According to another feature of the second aspect of the invention, the propagation velocity measuring apparatus further comprises means for determining a degree of arterial sclerosis based on the modified propagation-velocity value according to a predetermined relationship between degree of arterial sclerosis and modified propagation velocity.
According to a third aspect of the present invention, there is provided an apparatus for measuring a propagation velocity of a pulse wave which is propagated through an artery of a living subject, the apparatus comprising a blood-pressure measuring device which includes an inflatable cuff adapted to be wound around a body portion of the subject, a pulse-wave sensor which detects the pulse wave transmitted from the artery of the subject to the cuff when a pressure in the cuff is changed, and means for determining a blood pressure value of the subject based on the detected pulse wave, an electrocardiographic-waveform detecting device which includes a plurality of electrodes adapted to contact a body surface of the subject and which detects an electrocardiographic waveform from the subject through the electrodes, time-difference determining means for successively determining a time difference between a first periodic point relating to the detected electrocardiographic waveform and a second periodic point relating to the detected pulse wave, temporary-propagation-velocity determining means for successively determining a temporary propagation velocity of the pulse wave based on each of the successively determined time-difference values, judging means for judging whether the change of each of the successively determined temporary propagation-velocity values with respect to the change of the cuff pressure is smaller than a reference value, and proper-propagation-velocity determining means for determining a proper propagation velocity from at least one temporary propagation-velocity value for which a positive judgment is made by the judging means.
There is known a BP measuring device which includes an inflatable cuff adapted to be wound around a body portion of the subject, a pulse-wave sensor which detects the pulse wave transmitted from the artery of the subject to the cuff when a pressure in the cuff is changed, and means for determining a blood pressure value of the subject based on the detected pulse wave. The BP measuring device is disclosed in, e.g., U.S. patent application Ser. No. 08/273,929. In the case where the present invention is embodied with the BP measuring device, however, the (temporary) propagation-velocity values determined by the (temporary) propagation-velocity determining means gradually increase as the pressure of the cuff gradually decreases, when the cuff pressure is changed in a range above a mean BP value of the subject. In the pulse-wave propagation velocity measuring apparatus in accordance with the third aspect of the invention, the judging means judges whether the change of each of the determined temporary propagation-velocity values with respect to the change of the cuff pressure is smaller than a reference value, and the proper-propagation-velocity determining means determines a proper propagation velocity from at least one temporary propagation-velocity value for which a positive judgment is made by the judging means. Since the proper propagation-velocity value is determined from the temporary propagation value or values that is or are stable independent of the change of cuff pressure, the accuracy of measurement of pulse-wave propagation velocity is improved.
According to a preferred feature of the third aspect of the invention, the proper-propagation-velocity determining means comprises means for determining, as the proper propagation velocity, an average of a plurality of temporary propagation-velocity values for each of which the positive judgment is made by the judging means. The thus determined proper propagation velocity enjoys a higher accuracy than a proper propagation velocity as one of a plurality of temporary propagation-velocity values for each of which the positive judgment is made by the judging means. However, the proper-propagation-velocity determining means may comprise means for determining, as the proper propagation velocity, a temporary propagation-velocity values for which a positive judgment is made by the judging means.
According to another feature of the third aspect of the invention, the propagation velocity measuring apparatus further comprises proper-propagation-velocity modifying means for modifying the determined proper propagation-velocity value to a modified proper propagation-velocity value corresponding to a predetermined blood pressure value, based on the measured blood pressure value, according to a predetermined relationship between modified proper propagation velocity, and determined proper propagation velocity and measured blood pressure.
According to another feature of the third aspect of the invention, the propagation velocity measuring apparatus further comprises means for determining a degree of arterial sclerosis of the subject based on the modified proper propagation-velocity value according to a predetermined relationship between degree of arterial sclerosis and modified proper propagation velocity.
According to a fourth aspect of the present invention, there is provided an apparatus for measuring a propagation velocity of a pulse wave which is propagated through an artery of a living subject, the apparatus comprising a blood-pressure measuring device which includes an inflatable cuff adapted to be wound around a body portion of the subject, a pulse-wave sensor which detects a plurality of heartbeat-synchronous pulses of the pulse wave transmitted from the artery of the subject to the cuff when a pressure in the cuff is changed, and means for determining a blood pressure value of the subject based on variation of respective amplitudes of the detected heartbeat-synchronous pulses of the pulse wave, an electrocardiographic-waveform detecting device which includes a plurality of electrodes adapted to contact a body surface of the subject and which detects an electrocardiographic waveform from the subject through the electrodes, time-difference determining means for successively determining a time difference between a first periodic point relating to each of heartbeat-synchronous pulses of the detected electrocardiographic waveform and a second periodic point relating to a corresponding one of the detected heartbeat-synchronous pulses of pulse wave, propagation-velocity determining means for successively determining a propagation velocity of the pulse wave based on each of the successively determined time-difference values, judging means for judging whether the change of each of the successively determined propagation-velocity values with respect to the change of the cuff pressure is smaller than a reference value, and correcting means for correcting at least one first pulse of the heartbeat-synchronous pulses of the pulse wave which is deviated from at least one second pulse for which a positive judgment is made by the judging means, based on the deviation of the first pulse from the second pulse, so that the blood-pressure measuring device measures the blood pressure value of the subject based on the variation of the respective amplitudes of the heartbeat-synchronous pulses including the corrected first pulse.
In the case where the present invention is embodied with the BP measuring device which determines a BP value of the subject based on variation of respective amplitudes of heartbeat-synchronous pulses of the pulse wave obtained when the cuff pressure is changed, the relationship of correspondence of pulse amplitude and cuff pressure (or blood pressure) may break for a certain reason during each BP measuring operation of the BP measuring device. Hence, if a first pulse of the heartbeat-synchronous pulses of the pulse wave is deviated from one or more second pulses corresponding to one or more propagation-velocity values for each of which a positive judgment is made by the judging means, the correcting means corrects the first pulse, e.g., amplitude of the first pulse, or value of the cuff pressure at the time of detection of the first pulse, so that the BP measuring device measures a BP value of the subject based on the variation of respective amplitudes of heartbeat-synchronous pulses including the corrected first pulse. Thus, the accuracy of measurement of BP values is improved.
According to a preferred feature of the fourth aspect of the invention, the correcting means comprises estimating means for estimating, based on a plurality of the determined propagation-velocity values for each of which the positive judgment is made by the judging means, a next propagation-velocity value which will be determined by the propagation-velocity determining means, and difference determining means for determining a difference between the estimated next propagation-velocity value and an actual next propagation-velocity actually determined by the propagation-velocity determining means.
According to another feature of the fourth aspect of the invention, the correcting means further comprises means for determining a correction value based on the determined difference according to a predetermined expression, and adding the correction value to an amplitude of the first pulse. Thus, the first pulse is corrected.
According to another feature of the fourth aspect of the invention, the correcting means further comprises means for determining a correction value based on the determined difference according to a predetermined expression, and adding the correction value to a value of the cuff pressure at a time of detection of the first pulse. Thus, the first pulse is corrected.
According to a fifth aspect of the present invention, there is provided an apparatus for measuring a propagation velocity of a pulse wave which is propagated through an artery of a living subject, the apparatus comprising, a blood-pressure measuring device which includes an inflatable cuff adapted to be wound around a body portion of the subject, a pulse-wave sensor which detects the pulse wave transmitted from the artery of the subject to the cuff when a pressure in the cuff is changed, and means for determining a blood pressure value of the subject based on the detected pulse wave, an electrocardiographic-waveform detecting device which includes a plurality of electrodes adapted to contact a body surface of the subject and which detects an electrocardiographic waveform from the subject through the electrodes, time-difference determining means for determining a time difference between a first periodic point relating to the detected electrocardiographic waveform and a second periodic point on relating to the detected pulse wave, propagation-velocity determining means for determining a propagation velocity of the pulse wave based on the determined time-difference value, and terminating means for terminating a blood pressure measurement of the blood-pressure measuring device when the determined propagation-velocity value does not fall within a permission range.
In the case where the present invention is embodied with the BP measuring device which determines a BP value of the subject based on the pulse wave obtained when the cuff pressure is changed, the relationship of correspondence of pulse wave and cuff pressure (or blood pressure) may break for a certain reason during each BP measuring operation of the BP measuring device. Hence, if an abnormal event occurs during each BP measurement and the propagation velocity excessively changes, i.e., increases or decreases, the BP measurement is forcedly terminated. Thus, an inaccurate BP measurement or value is effectively avoided.
According to a preferred feature of the fifth aspect of the invention, the propagation velocity measuring apparatus further comprises a memory which stores data indicative of a plurality of the permission ranges corresponding to a plurality of values of the cuff pressure at which a plurality of propagation-velocity values are determined by the propagation-velocity determining means.
According to a sixth aspect of the present invention, there is provided an apparatus for measuring a propagation velocity of a pulse wave which is propagated through an artery of a living subject, the apparatus comprising, a first and a second heartbeat-synchronous-wave sensor which detect, as the pulse wave, a first and a second heartbeat-synchronous wave, respectively, each of which is produced from the artery of the subject in synchronism with a heartbeat of the subject, maximum-slope-line determining means for determining, with respect to at least one of the first and second waves, a maximum-slope line which passes through a maximum-slope point where a heartbeat-synchronous pulse of the one wave takes a maximum slope, such that the maximum-slope line has the maximum slope, base-line determining means for determining, with respect to the one wave, a base line which passes through respective minimum points on both sides of a maximum point of the heartbeat-synchronous pulse of the one wave, reference-point determining means for determining, as a reference point, a point of intersection of the maximum-slope line and the base line, and propagation-velocity determining means for determining the propagation velocity of the pulse wave based on a time difference between the reference point determined with respect to the heartbeat-synchronous pulse of the one wave and a corresponding heartbeat-synchronous pulse of the other of the first and second waves.
The pulse wave propagated through the artery of the subject contains, as a primary component, a travelling wave and additionally contains, as a secondary component, a reflected wave produced when the travelling wave is reflected by the wall of the artery, e.g., bifurcate wall, as illustrated in FIG. 25. Since, generally, the component of the reflected wave is weak relative to the component of the travelling wave, the maximum point of each travelling wave can be estimated as the maximum point of each pulse wave. However, as the arterial sclerosis becomes worse, the component of the reflected wave becomes stronger, and the maximum point of each pulse wave is more influenced by the component of the reflected wave, as shown in FIG. 29. If the maximum point of the pulse wave is used as a reference point for determining a pulse-wave propagation velocity, then the propagation velocity may largely change depending upon the degree of arterial sclerosis of the subject and/or the shape of the artery wall where the pulse wave is detected. In the pulse-wave propagation velocity measuring apparatus in accordance with the sixth aspect of the invention, however, the reference-point determining means determines, as the reference point, a point of intersection of the maximum-slope line and the base line. This reference point (TS) is free from the influences of the reflected wave, as illustrated in FIG. 25. Thus, the reference point does not change or move relative to the pulse wave even if the subject may have a serious arterial sclerosis. Accordingly, the present apparatus measures, with high accuracy, the propagation velocity of the pulse wave.
According to a seventh aspect of the present invention, there is provided an apparatus for measuring a propagation velocity of a pulse wave which is propagated through an artery of a living subject, the apparatus comprising, a first and a second heartbeat-synchronous-wave sensor which detect, as the pulse wave, a first and a second heartbeat-synchronous wave, respectively, each of which is produced from the artery of the subject in synchronism with a heartbeat of the subject, maximum-slope-line determining means for determining, with respect to at least one of the first and second waves, a maximum-slope line which passes through a maximum-slope point where a heartbeat-synchronous pulse of the one wave takes a maximum slope, such that the maximum-slope line has the maximum slope, maximum-point-line determining means for determining, with respect to the one wave, a maximum-point line which passes through a maximum point where the heartbeat-synchronous pulse of the one wave takes a maximum amplitude, such that the maximum-point line is parallel to a base line which passes through respective minimum points on both sides of the maximum point of the heartbeat-synchronous pulse of the one wave, reference-point determining means for determining a reference point from a point of intersection of the one wave and a line which passes through a point of intersection of the maximum-slope line and the maximum-point line and is perpendicular to the maximum-point line, and propagation-velocity determining means for determining the propagation velocity of the pulse wave based on a time difference between the reference point determined with respect to the heartbeat-synchronous pulse of the one wave and a corresponding heartbeat-synchronous pulse of the other of the first and second waves.
In the pulse-wave propagation velocity measuring apparatus in accordance with the seventh aspect of the invention, the reference-point determining means determines the reference point from a point of intersection of the one wave and a line which passes through a point of intersection of the maximum-slope line and the maximum-point line and is perpendicular to the maximum-point line. The reference point may be determined as being equal to the point of intersection of the one wave and the line or otherwise be determined based on the intersection point. This reference point (TS) is free from the influences of the reflected wave, as illustrated in FIG. 29. Thus, the reference point does not change or move relative to the pulse wave even if the subject may have a serious arterial sclerosis. Accordingly, the present apparatus measures, with high accuracy, the propagation velocity of the pulse wave.
According to an eighth aspect of the present invention, there is provided an apparatus for measuring a propagation velocity of a pulse wave which is propagated through an artery of a living subject, the apparatus comprising a first and a second heartbeat-synchronous-wave sensor which detect, as the pulse wave, a first and a second heartbeat-synchronous wave, respectively, each of which is produced from the artery of the subject in synchronism with a heartbeat of the subject, reference-point determining means for determining, with respect to at least one of the first and second waves, a maximum-slope point where a heartbeat-synchronous pulse of the one wave takes a maximum slope, and determining the determined maximum-slope point as a reference point, and propagation-velocity determining means for determining the propagation velocity of the pulse wave based on a time difference between the reference point determined with respect to the heartbeat-synchronous pulse of the one wave and a corresponding heartbeat-synchronous pulse of the other of the first and second waves.
In the pulse-wave propagation velocity measuring apparatus in accordance with the eighth aspect of the invention, the reference-point determining means determines, as the reference point, a maximum-slope point where the pulse wave takes a maximum slope. This reference point (Kmax) is free from the influences of the reflected wave, as illustrated in FIG. 25 or 29. Thus, the reference point does not change or move relative to the pulse wave even if the subject may have a serious arterial sclerosis. Accordingly, the present apparatus measures, with high accuracy, the propagation velocity of the pulse wave.